Primitive Molluscs
The remains are not preserved of those
primitive Molluscs in which we might see the protecting shell
gradually thickening, and deforming the worm-like body, we are
not without indications of the process. Two unequal branches of
the early wormlike organisms shrank into strong protective
shells. The lower branch became the Brachiopods; the more
advanced branch the Molluscs. In the Mollusc world, in turn,
there are several early types developed. In the Pelecypods (or
Lamellibranchs--the mussel, oyster, etc.) the animal retires
wholly within its fortress, and degenerates. The Gastropods
(snails, etc.) compromise, and retain a certain amount of
freedom, so that they degenerate less. The highest group, the
Cephalopods, "keep their heads," in the literal sense, and we
shall find them advancing from form to form until, in the octopus
of a later age, they discard the ancestral shell, and become the
aristocrats of the Mollusc kingdom.
The last and most important line that led upward from the chaos
of Archaean worms is that of the Arthropods. Its early
characteristic was the acquisition of a chitinous coat over the
body. Embryonic indications show that this was at first a
continuous shield, but a type arose in which the coat broke into
sections covering each segment of the body, giving greater
freedom of movement. The shield, in fact, became a fine coat of
mail. The Trilobite is an early and imperfect experiment of the
class, and the larva of the modern king-crab bears witness that
it has not perished without leaving descendants. How later
Crustacea increase the toughness of the coat by deposits of lime,
and lead on to the crab and lobster, and how one early branch
invades the land, develops air-breathing apparatus, and
culminates in the spiders and insects, will be considered later.
We shall see that there is most remarkable evidence connecting
the highest of the Arthropods, the insect, with a remote Annelid
ancestor.
We are thus not entirely without clues to the origin of the more
advanced animals we find when the fuller geological record
begins. Further embryological study, and possibly the discovery
of surviving primitive forms, of which Central Africa may yet
yield a number, may enlarge our knowledge, but it is likely to
remain very imperfect. The fossil records of the long ages during
which the Mollusc, the Crustacean, and the Echinoderm slowly
assumed their characteristic forms are hopelessly lost. But we
are now prepared to return to the record which survives, and we
shall find the remaining story of the earth a very ample and
interesting chronicle of evolution.
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